FLOOD05175

• County: Summit
• Community: Breckenridge
• Stream Name: Blue River
• Basin: Colorado Mainstem
• Title: Technical Addendum Floodplain Information Blue River
• Date: 12/1/1974
• Prepared For: Summit County
• Prepared By: Leonard Rice Consulting Water Engineers, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

- 24 -

HYDRAULICS

METHODOLOGY

Two hydraulic models were used for the flood plain definition.
The first was the routing capability contained in the MITCAT
model; the second was the Corps of Engineers HEC-2 model
which was .used for actual flood plain definition in the town
of Breckenridge.

The channel routing capability in MITCAT used an average
trapezoidal channel for each segment of the model. The peak
flow determined from the runoff portion of the model at each
upstream design point plus the overland contribution for that
particular catchment are routed through the representative
channel using Manning's formula. ThIs is for modeling the
effect of channel storage on the hydrograph. Following this
preliminary determination, the HEC-2.Corps of Engineers com-
puter model was used for actual determination of the flood-
stage and extent of flooding in the Blue River through Brecken-
ridge. Hand calculations were made at structures using the
weir formula and culvert coefficients.

It was found that the greatest flood potential for the town of
Breckenridge is from flows in the ,Blue River originating above
the Tarn Reservoir. The inflow hydrograph determined from the
hydrology study was routed through the Tarn Reservoir to give
a peak discharge out of the Tarn spillway. This peak discharge
of the design hydrograph was used in HEC-2 for the reach be-
tween Broken Lance Road (U/S) and Cemetery Road. A copy of
the inflow and outflow routing hydrograph is shown in Figure
10. Table 5 is the actual computer printout for the IRF storm.

Cross sections for use in the HEC-2 program were taken from a
combination of the base mapping provided by the town of Brecken-
ridge as well as actual field cross sections taken in the
vicinity of crossing structures. Field confirmation was also
made of the base mapping to insure accuracy. Base mapping was
taken from 1971 through 1973 aerial photography. One area in
the vicinity of Village Road was found to vary considerably
from base mapping and new topographic mapping was provided.
Other recent changes did not appreciably alter the floodplain
definition.

"Mannings" values were selected on the basis of average channel
reaches. These varied from .035 to .045 within the channel to
.045 to .055 on the overbank. The average slope of the Blue

~ Leonard Rice Consulting Water Engine€rs.lnc.